1. Field of the Invention
The present invention relates to a control unit of a power converter for converting a direct current to an AC-phase voltage having at least 3 electric potentials and more particularly to improvement of switching control of elements constituting the power converter.
2. Prior Art
A dipolar modulation method for outputting positive and negative pulse voltages alternately via zero voltage as wave form improvement of a three-level inverter has been proposed in "A Novel approach to the Generation and Optimization of Three-level PWM Wave Forms", pp. 1255 to 1262. The paper describes that the output wave form can be improved by modifying the relationship between bias and amplitude of a modulation wave as predetermined.
When dipolar modulation is performed on the basis of the relationship described in the above prior art, it is found that the DC side elements (two switching elements connected to the DC power source) and AC side elements (two switching elements connected to the AC output terminal) are not uniform in the loss generated in the switching elements. The heat generation distribution of the DC side elements and AC side elements is not uniform due to this lack of uniformity, and the element cooling system is required to be designed according to the elements with a large calorific value, and the inverter system becomes large.